A. Field
The present invention concerns an improved device for adjusting the flow rate of a mobile oil-injected screw-type compressor.
B. Related Art
In particular, the present invention concerns an improved device for adjusting the flow rate of mobile oil-injected screw-type compressors which are driven by a thermal motor and which can typically provide operating pressures from 5 to 35 bar, whereby also the supplied flow rate of compressed gas can be adjusted in a sliding manner between 0 and 100%.
Such devices for adjusting the flow rate of a mobile oil-injected screw-type compressor which are driven by a thermal motor are already known, whereby the screw-type compressor is provided with an inlet and with an outlet onto which is connected a pressure vessel with an outlet pipe for supplying a compressed gas and whereby the device mainly consists of a control valve which is connected with its input to the pressure vessel via a pressure pipe and which, at its output, as of a certain pre-determined value of the pressure in the pressure pipe of the pressure vessel, supplies a control pressure which is in proportion to said pressure in the pressure pipe of the pressure vessel; an electronic speed controller for adjusting the rotational speed of the motor which is connected to the above-mentioned control pressure of the control valve via a pressure sensor and a first control line and which is such that, as the control pressure rises, the motor is set at a lower rotational speed; and of a pneumatically controlled inlet valve on the inlet of the compressor, which inlet valve consists of a housing in which a valve element can be shifted to and fro in the axial direction between an open and a closed position and which is sealed on one side of the valve element so as to form a pressure chamber which is connected to the control pressure of the control valve via a second control line.
With most known devices for adjusting the flow rate of a mobile oil-injected screw-type compressor, the valve element of the inlet valve of the compressor is moreover pushed in an open position by means of a compression spring during start up.
A disadvantage of these known devices for adjusting the flow rate of a mobile oil-injected screw-type compressor is that, during a cold start up, there is not enough torque.
This is due to the fact that the inlet valve, during the start up, is pushed in an open position by the compression spring, such that while the screw-type compressor increases speed from a standstill up to the required minimum rotational speed, air is drawn in and compressed.
The compression of air hinders the screw-type compressor in gaining its rotational speed, and that is why a high torque is required.
With other known devices, this low torque problem during a cold start up is remedied by keeping the inlet valve in a closed position during start up until the screw-type compressor has reached the required minimum rotational speed.
A disadvantage of these known devices, however, is that they consume a lot and consequently are not economical, so that refueling is often required, which is time-consuming and laborious.
The present invention aims to remedy one or several of the above-mentioned and other disadvantages in a simple manner.